Take the two secondaries and connect the "neutral" of one secondary and the "live" of the other secondary and tada...
A centre tapped trannie.
If I've offended anyone with the terminolagy please let me know how to phrase it.
---- || ----- +
) || (
) || ( Secondary A
) || (
) || ------ -
) || ------ +
) ||(
) ||( Secondary B
) ||(
---- || ------ -
In other words. Connect the - of Secondary A and the + of secondary B.
To check If you've got the phase correct. Put a VOM accros the + of Sec. A and the - of Sec B. If it reads +- 0V then one of the secondaries is the wrong way round. If it reads the sum of both secondaries, it's right.
There is some other threads dealing with this. Look at them too.
A centre tapped trannie.
If I've offended anyone with the terminolagy please let me know how to phrase it.
---- || ----- +
) || (
) || ( Secondary A
) || (
) || ------ -
) || ------ +
) ||(
) ||( Secondary B
) ||(
---- || ------ -
In other words. Connect the - of Secondary A and the + of secondary B.
To check If you've got the phase correct. Put a VOM accros the + of Sec. A and the - of Sec B. If it reads +- 0V then one of the secondaries is the wrong way round. If it reads the sum of both secondaries, it's right.
There is some other threads dealing with this. Look at them too.
Re: Re: Dual rail supply, no centre tap
Why? Such a transformer has two secondaries two. It is just that they have tied two ends together, so you don't see it. It is better to have access to both secondaries, since it gives more flexibility. It makes it possible to use a better grounding scheme, for instance.
Of course, if using it as a center tap one, it is important to tie the right two ends together, but as long as they belong to two different secondaries, the worst that can happen is that the two secondaries cancel so you get zero volts in total.
ezkcdude said:
Why? Such a transformer has two secondaries two. It is just that they have tied two ends together, so you don't see it. It is better to have access to both secondaries, since it gives more flexibility. It makes it possible to use a better grounding scheme, for instance.
Of course, if using it as a center tap one, it is important to tie the right two ends together, but as long as they belong to two different secondaries, the worst that can happen is that the two secondaries cancel so you get zero volts in total.
AndrewT said:
You got me. Now you see why I didn't want to use a transformer for my own PSU project. Anyway, I did find a link which is helpful (to me, at least) from Hammond:
http://www.hammondmfg.com/5CHook.htm
Hi Ez,
Hammond is a nice link. Unfortunately the Hammond page does not alert the user to the possibility of incorrect wiring consequences.
The first diagram showing the series primary connections shows the correct way to wire them for, usually, 240Vac supply.
If 2 to 4 or 1 to 3 were connected instead of 2 to 3 or 1 to 4 then the transformer goes into almost instantaneous destruction mode.
Note, there are four ways to do the primary series connection and a 50% chance of blowing the transformer. The plug top fuse (UK) and/or the distribution board fuse should prevent more serious house damage.
Since finding the light bulb trick, here on the Forum, I never fail to use it for all my maintenance switch on. It will save the transformer from all miswiring errors and may save the downstream semiconductors from terminal damage as well.
Hammond is a nice link. Unfortunately the Hammond page does not alert the user to the possibility of incorrect wiring consequences.
The first diagram showing the series primary connections shows the correct way to wire them for, usually, 240Vac supply.
If 2 to 4 or 1 to 3 were connected instead of 2 to 3 or 1 to 4 then the transformer goes into almost instantaneous destruction mode.
Note, there are four ways to do the primary series connection and a 50% chance of blowing the transformer. The plug top fuse (UK) and/or the distribution board fuse should prevent more serious house damage.
Since finding the light bulb trick, here on the Forum, I never fail to use it for all my maintenance switch on. It will save the transformer from all miswiring errors and may save the downstream semiconductors from terminal damage as well.
Hi!
I have a strange phenomenon on my dual rail transformer:
It is a 12VA 2x7.5Vac EI core transformer. Both rail has exactly the same voltage.
I wanted to make a "center tap" for my dual rail supply, but when I tie one secondary's "start" and the other's "end" together (like on the post 3) I get less voltage than the expected double voltage! I measured the voltage between the two "center" wire (the - of Secondary A and the + of secondary B) and it has 2.5Vac instead of zero!!!
Could you tell anyone, how can this be?
greets:
Tyimo
I have a strange phenomenon on my dual rail transformer:
It is a 12VA 2x7.5Vac EI core transformer. Both rail has exactly the same voltage.
I wanted to make a "center tap" for my dual rail supply, but when I tie one secondary's "start" and the other's "end" together (like on the post 3) I get less voltage than the expected double voltage! I measured the voltage between the two "center" wire (the - of Secondary A and the + of secondary B) and it has 2.5Vac instead of zero!!!
Could you tell anyone, how can this be?
greets:
Tyimo
I'm guessing you disconnected the wires you planned to use as a centre tap again and measured between the two? If so, that's no good because that way, both windings are floating with respect to eachother.
Please measure both secondary windings seperately and report the results back to us.
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The mystery is solved!
My voltmeter played a trick! I connected the vires and now I see double voltage on the ends and zero in the middle.
One more question: this 12VA transformer's nominal voltage is 2x 7.5Vac, but I measure without load 9.5Vac. How much will fall back to partial load? ( on 0.18A)
Thanks for all the helps!
greets:
Tyimo
My voltmeter played a trick! I connected the vires and now I see double voltage on the ends and zero in the middle.
One more question: this 12VA transformer's nominal voltage is 2x 7.5Vac, but I measure without load 9.5Vac. How much will fall back to partial load? ( on 0.18A)
Thanks for all the helps!
greets:
Tyimo
all transformers give more voltage when no load is connected than when the full rated current is drawn through a resistive dummy load.
This is defined by the transformer regulation.
Big transformer generally have a low regulation and small transformer have a bigger regulation. Typically we see regulation values of 3% to 10% in the transformer datasheets.
Very small transformer have enormous regulation. It can exceed 30%.
Your 12VA EI could be around 15% to 20%
i.e. the no load output voltage / rated output voltage = 1.20 = 1+(20%) = 20% regulation.
your 7.5Vac could measure 20% higher when fed with rated primary voltage. i.e. 9Vac
Then instead feed the primary with +6% (a common maximum tolerance for mains power) and that becomes 9.54Vac, from a 7.5Vac transformer.
YOU as designer must take account of the whole project's variables and that includes the mains transformer.
Instead you can be a "Builder" with no design input and that limits you to copying exactly a known and proven build guide for a successful copy of the project.
This is defined by the transformer regulation.
Big transformer generally have a low regulation and small transformer have a bigger regulation. Typically we see regulation values of 3% to 10% in the transformer datasheets.
Very small transformer have enormous regulation. It can exceed 30%.
Your 12VA EI could be around 15% to 20%
i.e. the no load output voltage / rated output voltage = 1.20 = 1+(20%) = 20% regulation.
your 7.5Vac could measure 20% higher when fed with rated primary voltage. i.e. 9Vac
Then instead feed the primary with +6% (a common maximum tolerance for mains power) and that becomes 9.54Vac, from a 7.5Vac transformer.
YOU as designer must take account of the whole project's variables and that includes the mains transformer.
Instead you can be a "Builder" with no design input and that limits you to copying exactly a known and proven build guide for a successful copy of the project.
Last edited:
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